Large particle size sodium nitrate multi-stage flash cooling crystallization production system and production process

Abstract

The present application relates to a kind of large particle size sodium nitrate multistage flash cooling crystallization production systems, it includes feed unit, evaporation unit, seed unit, flash crystallization unit, condensation unit, thickening unit, centrifugal drying unit and mother liquor reflux unit, seed unit includes material pump and seed tank, the discharge port of separator is connected to seed tank by material pump;Flash crystallization unit includes first flash crystallization ware, first material pump, second flash crystallization ware, second material pump, third flash crystallization ware and third material pump, the discharge port of seed tank is connected to the feed port of first flash crystallization ware, first flash crystallization ware, second flash crystallization ware, third flash crystallization ware are sequentially connected.The sodium nitrate solution of the present application is distributed uniformly by multistage flash, crystal growth time, and crystal size is adjusted by controlling fine crystal elimination amount, crystal size is large, particle size distribution is uniform, crystal size D90≥0.9mm, variation coefficient CV≤25.

Description

Technical Field

[0001] This invention belongs to the field of chemical raw material preparation technology, and mainly relates to a multi-stage flash evaporation and cooling crystallization production system and process for large-particle sodium nitrate. Background Technology

[0002] Sodium nitrate is an important chemical raw material, used as a defoamer and decolorizing agent in glass, a nitrating agent in dye intermediates, and as a fertilizer in agriculture. The main production methods for sodium nitrate are absorption, metathesis, and ion exchange. All these processes require concentration, crystallization, separation, and drying of the sodium nitrate solution at the end to produce high-purity sodium nitrate crystals. Currently, most existing sodium nitrate crystal production uses evaporation crystallization, which involves high system temperatures, strong corrosiveness to equipment, and difficulty in controlling the sodium nitrate particle size. Another method uses indirect cooling crystallization, producing large-particle-size sodium nitrate crystals, but this results in severe scaling on the heat exchanger's inner wall, requiring frequent cleaning cycles and hindering continuous production, leading to frequent shutdowns and increased operating costs. To address these problems, this invention provides a multi-stage flash evaporation cooling crystallization process that produces sodium nitrate crystals with a large average diameter, continuous operation, and easily controllable parameters. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of existing technology in processing efficiency and to provide a multi-stage flash evaporation and cooling crystallization production system for large-particle sodium nitrate. This method can achieve stable production of large-particle, low-distribution sodium nitrate crystals with high efficiency and solve the problems of large amount of wet crystal mother liquor entrainment and easy crystal agglomeration.

[0004] The technical problem solved by this invention is achieved through the following technical solution:

[0005] A multi-stage flash cooling crystallization production system for large-particle sodium nitrate includes a feeding unit, an evaporation unit, a seed crystallization unit, a flash crystallization unit, a condensation unit, a thickening unit, a centrifugal drying unit, and a mother liquor reflux unit. The feeding unit includes a feeding tank and a feeding pump. The evaporation unit includes an evaporator and an evaporation circulation pump. The feeding tank is connected to the inlet of the evaporator via the feeding pump and the evaporation circulation pump; the outlet of the evaporator is connected to the inlet of the separator. The seed crystallization unit includes a transfer pump and a seed crystallizer. The outlet of the separator is connected to the seed crystallizer via the transfer pump. The flash crystallization unit includes a first flash crystallizer, a first transfer pump, a second flash crystallizer, a second transfer pump, a third flash crystallizer, and a third transfer pump. The outlet of the seed crystallizer is connected to the inlet of the first flash crystallizer, and the outlet of the first flash crystallizer is connected to the mother liquor reflux unit via the first transfer pump. The feed inlet of the second flash crystallizer is connected to the discharge outlet of the second flash crystallizer via a second transfer pump to the third flash crystallizer. The discharge outlet of the third flash crystallizer is connected to the feed inlet of the thickener in the thickening unit via a third transfer pump. The discharge outlet of the thickener is connected to the first centrifuge. The washing centrifuge unit includes a washing tank, a second centrifuge, and a dryer. The discharge outlet of the first centrifuge is connected to the washing tank, the discharge outlet of the washing tank is connected to the second centrifuge, and the discharge outlet of the second centrifuge is connected to the dryer. The mother liquor reflux unit includes a mother liquor tank and a reflux pump. The reflux port of the second centrifuge is connected to the mother liquor tank. The mother liquor tank is connected to the second and third flash crystallizers of the flash crystallizer unit via the reflux pump. The gas outlets of the first, second, and third flash crystallizers are connected to the condensation unit.

[0006] Furthermore, the condensation unit is a condenser, and the gas outlets of the evaporation chambers of the first flash crystallizer, the second flash crystallizer, and the third flash crystallizer are respectively equipped with a first pressure regulating valve, a second pressure regulating valve, and a third pressure regulating valve, which are connected to the condenser via the first pressure regulating valve, the second pressure regulating valve, and the third pressure regulating valve.

[0007] Furthermore, the first flash crystallizer is provided with a first fine crystal outlet, which is connected to a first fine crystal eliminator and then connected back to the first flash crystallizer via a first fine crystal eliminator pump.

[0008] Furthermore, the second flash crystallizer is provided with a second fine crystal outlet, which is connected to a second fine crystal eliminator and then connected back to the second flash crystallizer via a second fine crystal eliminator pump.

[0009] Furthermore, the third flash crystallizer is equipped with a third fine crystal outlet, which is connected to a third fine crystal eliminator and then connected back to the third flash crystallizer via a third fine crystal eliminator pump.

[0010] Moreover, the first flash crystallizer, the second flash crystallizer, and the third flash crystallizer are all DTB crystallizers.

[0011] A multi-stage flash evaporation and cooling crystallization process for producing large-particle sodium nitrate includes the following steps:

[0012] 1) Heating and evaporating sodium nitrate solution: The feed tank is connected to the feed pump. The feed pump delivers the raw liquid in the feed tank into the circulation pipeline connected between the evaporation circulation pump and the separator. The evaporation circulation pump delivers the mixed liquid into the evaporator for heating. The heated sodium nitrate solution is returned to the separator for evaporation and concentration to 62-65%.

[0013] 2) Seed crystals: The transfer pump transports the concentrated sodium nitrate solution in the above-mentioned circulation pipeline to the seed crystal tank. The seed crystal tank is equipped with a stirrer and continuously adds a certain amount of small seed crystals of a certain particle size. The seed crystals have a particle size of 20-50 μm and the amount added is 0.2-0.4% of the mass of the above-mentioned concentrated sodium nitrate solution. The residence time of the seed crystals in the seed crystal tank is 0.5-1 h.

[0014] 3) First-stage flash crystallization: The suspension in the seed tank is transported to the first flash crystallizer via pressure difference. The evaporation temperature in the first flash crystallizer is 60-70℃, the evaporation pressure is 20-32kPa, and the suspension density is 100-150kg / m³. 3 The sodium nitrate solution in the first flash crystallizer has a relative supersaturation of 1–1.5%.

[0015] 4) Two-stage flash crystallization: The crystal slurry in the first flash crystallizer is fed into the second flash crystallizer via the first transfer pump; the evaporation temperature in the second flash crystallizer is 50-60℃, the evaporation pressure is 12-20kPa, and the suspension density is 150-250kg / m³. 3 The sodium nitrate solution in the second flash crystallizer has a relative supersaturation of 1.5–2.5%.

[0016] 5) Three-stage flash crystallization: The crystal slurry in the second flash crystallizer is fed into the third flash crystallizer via the second transfer pump. The evaporation temperature in the third flash crystallizer is 40–50℃, the evaporation pressure is 7.5–12 kPa, and the suspension density is 250–400 kg / m³. 3 The relative supersaturation of the sodium nitrate solution in the third flash crystallizer was 2.5–4.0%.

[0017] 6) Thickening and separation: The crystal slurry in the third flash crystallizer is transported to the thickener by the third transfer pump. After thickening, the crystal slurry in the thickener flows into the first centrifuge by gravity. The mother liquor from the thickener and the first centrifuge flows into the mother liquor tank by gravity.

[0018] 7) Centrifugal drying unit: After the crystal slurry is dehydrated in the first centrifuge, the moisture content of the wet salt is 3-4%. The wet salt is conveyed to the washing tank by the screw feeder and washed with the raw material liquid for 0.25-0.5 hours. After washing, the crystal slurry flows into the second centrifuge for dehydration. After dehydration, the wet salt is sent to the dryer for drying. The dried sodium nitrate crystals have a D90 > 0.9 mm and a coefficient of variation CV ≤ 25.

[0019] 8) Mother liquor reflux: The washing mother liquor flows by gravity into the feed tank. A portion of the mother liquor stored in the tank is refluxed back to the second and third flash crystallizers via a reflux pump, while the remaining portion is refluxed back to the feed tank. A mother liquor discharge pipeline is installed, with the following flow rates: 20-25% for the second flash crystallizer, 40-45% for the third flash crystallizer, 30-35% for the feed tank, and 0-5% for the external discharge.

[0020] Furthermore, the gas outlets of the evaporation chambers in the first, second, and third flash crystallizers are all connected to the condenser through gas phase pipelines. A first pressure regulating valve, a second pressure regulating valve, and a third pressure regulating valve are respectively installed in the gas phase pipelines to adjust the evaporation pressure in the flash crystallizers step by step.

[0021] Furthermore, the first flash crystallizer has a clarification zone, and the first fine crystal eliminator is connected to the clarification zone of the first flash crystallizer. The suspension containing fine crystals in the clarification zone is heated and melted by the first fine crystal eliminator and then returned to the first flash crystallizer by the first fine crystal eliminator pump. The particle size of the fine crystals entering the first fine crystal eliminator from the clarification zone of the first flash crystallizer is no greater than 30 μm, and the fine crystal elimination cycle ratio R = 2 to 3.

[0022] The second flash crystallizer has a clarification zone, and the second fine crystal eliminator is connected to the clarification zone of the second flash crystallizer. The suspension containing fine crystals in the clarification zone is heated and melted by the second fine crystal eliminator and then returned to the second flash crystallizer by the second fine crystal eliminator pump. The particle size of the fine crystals entering the second fine crystal eliminator from the clarification zone of the second flash crystallizer is no greater than 50 μm, and the fine crystal elimination circulation ratio R = 3 to 3.5.

[0023] The third flash crystallizer has a clarification zone, and the third fine crystal eliminator is connected to the clarification zone of the third flash crystallizer. The suspension containing fine crystals in the clarification zone is heated and melted by the third fine crystal eliminator and then returned to the third flash crystallizer by the third fine crystal eliminator pump. The particle size of the fine crystals entering the third fine crystal eliminator 17 from the clarification zone of the third flash crystallizer is no greater than 80 μm, and the fine crystal elimination circulation ratio R = 3.5~5.

[0024] The advantages and beneficial effects of this invention are as follows:

[0025] 1. The large-particle sodium nitrate multi-stage flash evaporation cooling crystallization production system and process of the present invention, compared with the sodium nitrate obtained by evaporation crystallization or single-stage cooling crystallization methods with a main particle size distribution of 0.3-0.6 mm, can prepare sodium nitrate crystals with D90>0.9 mm using the method of the present invention. The particle size of the sodium nitrate crystals is 1.5-3 times that of the former; the coefficient of variation CV≤25; and the product has good crystal fluidity and does not clump after drying.

[0026] 2. The large-particle sodium nitrate multi-stage flash cooling crystallization production system and process of the present invention, after the sodium nitrate solution is concentrated to saturation, in order to prevent explosive nucleation, an appropriate amount of seed crystals should be added before crystallization nucleation. Essentially, it provides sufficient crystal surface area for crystal growth before crystal nucleation. In the present invention, the added sodium nitrate seed crystals have a particle size of 20-50 μm and the amount added is 0.2-0.4% of the mass of the above-mentioned concentrated sodium nitrate solution.

[0027] 3. In the large-particle-size sodium nitrate multi-stage flash cooling crystallization production system and process of the present invention, the relative supersaturation of the sodium nitrate crystallization process should be controlled within the metastable region to promote crystal growth rather than large-scale nucleation. The relative supersaturation of the sodium nitrate solution in the first flash crystallizer is 1-1.5%, in the second flash crystallizer it is 1.5-2.5%, and in the third flash crystallizer it is 2.5-4.0%. In the present invention, if the relative supersaturation is lower than the lower limit of the above-mentioned flash cooling crystallization process, it will cause a significant decline in the growth rate of sodium nitrate crystals. If it is higher than the upper limit of the above-mentioned relative supersaturation, there will be a risk of heterogeneous nucleation, resulting in a large amount of crystal precipitation or even equipment blockage.

[0028] 4. In the large-particle-size sodium nitrate multi-stage flash cooling crystallization production system and process of the present invention, the supersaturation of the solution increases progressively during the three-stage continuous flash cooling crystallization process. Therefore, sufficient crystal surface area should be provided in each stage of continuous crystallization to allow for the growth of supersaturated solute in the solution, thereby increasing the degree of supersaturation relief in the discharge. The suspension density in the first flash crystallizer is 100-150 kg / m³. 3 The suspension density in the second flash crystallizer is 150–250 kg / m³. 3 The suspension density in the third flash crystallizer is 250–400 kg / m³. 3 According to the present invention, based on the relative supersaturation and corresponding suspension density of the three-stage flash cooling crystallization, large-particle sodium nitrate crystals with a main particle size >900μm can be stably produced at the third-stage discharge.

[0029] 5. The large-particle sodium nitrate multi-stage flash cooling crystallization production system and process of the present invention have strong applicability, do not use heat exchangers, and avoid scaling on the heat exchanger walls; an appropriate amount of seed crystals are introduced before flash cooling crystallization to ensure uniform crystal growth under high supersaturation; after multi-stage flash evaporation of the sodium nitrate solution, the crystal growth time is uniformly distributed, and the crystal particle size is adjusted by controlling the amount of fine crystal removal, resulting in large crystal particle size, uniform particle size distribution, crystal particle size D90≥0.9mm, and coefficient of variation CV≤25. Attached Figure Description

[0030] Figure 1 This is a flowchart of the large-particle sodium nitrate multi-stage flash evaporation and cooling crystallization production system of the present invention.

[0031] 1-Feed tank, 2-Feed pump, 3-Evaporator, 4-Evaporation circulation pump, 5-Separator, 6-Transfer pump, 7-Seed tank, 8-First flash crystallizer, 9-First fine crystal eliminator, 10-First fine crystal eliminator pump, 11-First transfer pump, 12-Second flash crystallizer, 13-Second fine crystal eliminator, 14-Second fine crystal eliminator pump, 15-Second transfer pump, 16-Third flash crystallizer, 17-Third fine crystal eliminator, 18-Third fine crystal eliminator pump, 19-Third transfer pump, 20-First pressure regulating valve, 21-Second pressure regulating valve, 22-Third pressure regulating valve, 23-Condenser, 24-Thickener, 25-First centrifuge, 26-Washing tank, 27-Second centrifuge, 28-Dryer, 29-Mother liquor tank, 30-Reflux pump. Detailed Implementation

[0032] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0033] A multi-stage flash evaporation and cooling crystallization production system for large-particle sodium nitrate includes a feeding unit, an evaporation unit, a seed crystal unit, a flash crystallization unit, a condensation unit, a thickening unit, a centrifugal drying unit, and a mother liquor reflux unit.

[0034] The feeding unit includes a feeding tank 1 and a feeding pump 2. The evaporation unit includes an evaporator 3 and an evaporation circulation pump 4. The feeding tank 1 is connected to the inlet of the evaporator 3 via the feeding pump 2 and the evaporation circulation pump 4. The outlet of the evaporator 3 is connected to the inlet of the separator 5. The seed crystal unit includes a transfer pump 6 and a seed crystal tank 7. The outlet of the separator 5 is connected to the seed crystal tank 7 via the transfer pump 6.

[0035] The flash crystallization unit includes a first flash crystallizer 8, a first transfer pump 11, a second flash crystallizer 12, a second transfer pump 15, a third flash crystallizer 12, and a third transfer pump 15. The outlet of the seed tank 7 is connected to the inlet of the first flash crystallizer 8. The outlet of the first flash crystallizer 8 is connected to the inlet of the second flash crystallizer 12 via the first transfer pump 11, and the outlet of the second flash crystallizer 12 is connected to the third flash crystallizer 16 via the second transfer pump 15.

[0036] The first flash crystallizer 8 is connected to the clarification zone by a first fine crystal outlet, which is connected to the first fine crystal eliminator 9 and then back to the first flash crystallizer 8 via a first fine crystal eliminator pump 10. The second flash crystallizer 12 is connected to the clarification zone by a second fine crystal outlet, which is connected to the second fine crystal eliminator 13 and then back to the second flash crystallizer 12 via a second fine crystal eliminator pump 14. The third flash crystallizer 16 is connected to the clarification zone by a second fine crystal outlet, which is connected to the third fine crystal eliminator 17 and then back to the second flash crystallizer 12 via a third fine crystal eliminator pump 18. All three flash crystallizers (first, second, and third) are DTB crystallizers.

[0037] The condensation unit is a condenser. The gas outlets of the evaporation chambers of the first flash crystallizer 8, the second flash crystallizer 12, and the third flash crystallizer 16 are respectively equipped with a first pressure regulating valve 20, a second pressure regulating valve 21, and a third pressure regulating valve 22, which are connected to the condenser. The evaporation pressure in the flash crystallizer is adjusted step by step.

[0038] The outlet of the third flash crystallizer 16 is connected to the inlet of the thickener 24 of the thickening unit via the third transfer pump 19. The outlet of the thickener 24 is connected to the first centrifuge 25. The washing and centrifuging unit includes a washing tank 26, a second centrifuge 27, and a dryer 28. The outlet of the first centrifuge 25 is connected to the washing tank 26, the outlet of the washing tank 26 is connected to the second centrifuge 27, and the outlet of the second centrifuge 27 is connected to the dryer 28. The mother liquor reflux unit includes a mother liquor tank 29 and a reflux pump 30. The reflux port of the second centrifuge 27 is connected to the mother liquor tank 29. The mother liquor tank 29 is connected to the first / second / third flash crystallizer 16 of the flash crystallizer unit via the reflux pump 30. The gas outlets of the first flash crystallizer 8, the second flash crystallizer 12, and the third flash crystallizer 16 are connected to the condensation unit.

[0039] A multi-stage flash evaporation and cooling crystallization process for producing large-particle sodium nitrate includes the following steps:

[0040] 1) Heating and evaporating sodium nitrate solution: Feed tank 1 is connected to feed pump 2. Feed pump 2 transports the raw material liquid in feed tank 1 into the circulation pipeline connected between evaporation circulation pump 4 and separator 5. Evaporation circulation pump 4 transports the mixed liquid into evaporator 3 for heating. The heated sodium nitrate solution is then evaporated and concentrated to 62-65% in separator 5.

[0041] 2) Seed crystals: The transfer pump 6 transports the concentrated sodium nitrate solution in the above-mentioned circulation pipeline to the seed crystal tank 7. The seed crystal tank 7 is equipped with a stirrer and continuously adds a certain amount of small seed crystals of a certain particle size. The seed crystals have a particle size of 20-50 μm and the amount added is 0.2-0.4% of the mass of the above-mentioned concentrated sodium nitrate solution. The residence time of the seed crystals in the seed crystal tank is 0.5-1 h.

[0042] 3) First-stage flash crystallization: The suspension in the seed tank 7 is transported to the first flash crystallizer 8 by pressure difference. The first flash crystallizer 8 has an evaporation temperature of 60-70℃, an evaporation pressure of 20-32kPa, and a suspension density of 100-150kg / m³. 3 The sodium nitrate solution in the first flash crystallizer has a relative supersaturation of 1–1.5%.

[0043] 4) Two-stage flash crystallization: The crystal slurry in the first flash crystallizer 8 is fed into the second flash crystallizer 12 via the first transfer pump 11; the evaporation temperature of the second flash crystallizer 12 is 50-60℃, the evaporation pressure is 12-20kPa, and the suspension density is 150-250kg / m³. 3 The sodium nitrate solution in the second flash crystallizer has a relative supersaturation of 1.5–2.5%.

[0044] 5) Three-stage flash crystallization: The crystal slurry in the second flash crystallizer 12 is fed into the third flash crystallizer 16 via the second transfer pump 15. The evaporation temperature of the third flash crystallizer 16 is 40-50℃, the evaporation pressure is 7.5-12kPa, and the suspension density is 250-400kg / m³. 3 The relative supersaturation of the sodium nitrate solution in the third flash crystallizer 16 was 2.5–4.0%.

[0045] 6) Thickening and separation: The crystal slurry in the third flash crystallizer 16 is transported to the thickener 24 by the third transfer pump 19. After thickening, the crystal slurry in the thickener 24 flows into the first centrifuge 25. The mother liquor from the thickener 24 and the first centrifuge 25 flows into the mother liquor tank 29.

[0046] 7) Centrifugal drying unit: After the crystal slurry is dehydrated by the first centrifuge 25, the moisture content of the wet salt is 3-4%. The wet salt is conveyed to the washing tank 26 by a screw feeder and washed with the raw material liquid for 0.25-0.5 hours. After washing, the crystal slurry flows into the second centrifuge 27 for dehydration. After dehydration, the wet salt is sent to the dryer 28 for drying. The dried sodium nitrate crystals are 0.9-1.3 mm in size and have a coefficient of variation CV ≤ 25.

[0047] 8) Mother liquor reflux: The washing mother liquor flows by gravity into the feed tank 1. The mother liquor stored in the mother liquor tank 29 is refluxed by the reflux pump 30, with a portion of the mother liquor being refluxed to the second flash crystallizer 12 and the third flash crystallizer 16 respectively, and the other portion of the mother liquor being refluxed back to the feed tank 1. A mother liquor discharge pipeline is set up, with the mother liquor reflux rate of the second flash crystallizer 12 being 20-25%, the mother liquor reflux rate of the third flash crystallizer 16 being 40-45%, the mother liquor reflux rate of the feed tank being 30-35%, and the mother liquor discharge rate being 0-5%.

[0048] The gas outlets of the evaporation chambers in the first, second, and third flash crystallizers are all connected to the condenser 23 through gas phase pipelines. A first pressure regulating valve 20, a second pressure regulating valve 21, and a third pressure regulating valve 22 are respectively installed in the gas phase pipelines to adjust the evaporation pressure in the flash crystallizers step by step.

[0049] The first flash crystallizer 8 has a clarification zone, and the first fine crystal eliminator 9 is connected to the clarification zone of the first flash crystallizer 8. The suspension containing fine crystals in the clarification zone is heated and melted by the first fine crystal eliminator 9 and then returned to the first flash crystallizer 8 by the first fine crystal elimination pump 10. The particle size of the fine crystals entering the first fine crystal eliminator 9 from the clarification zone of the first flash crystallizer 8 is no greater than 30 μm, and the fine crystal elimination circulation ratio R = 2 to 3.

[0050] The second flash crystallizer 12 has a clarification zone. The second fine crystal eliminator 13 is connected to the clarification zone of the second flash crystallizer 12. The suspension containing fine crystals in the clarification zone is heated and melted by the second fine crystal eliminator 13 and then returned to the second flash crystallizer 12 by the second fine crystal elimination pump 14. The particle size of the fine crystals in the clarification zone of the second flash crystallizer 8 entering the second fine crystal eliminator 13 is no greater than 50 μm. The fine crystal elimination circulation ratio R = 3 to 3.5.

[0051] The third flash crystallizer 16 has a clarification zone. The third fine crystal eliminator 17 is connected to the clarification zone of the third flash crystallizer 16. The suspension containing fine crystals in the clarification zone is heated and melted by the third fine crystal eliminator 17 and then returned to the third flash crystallizer 16 by the third fine crystal elimination pump 18. The particle size of the fine crystals entering the third fine crystal eliminator 17 from the clarification zone of the third flash crystallizer 16 is no greater than 80 μm. The fine crystal elimination cycle ratio R = 3.5 to 5. The experimental data of sodium nitrate crystal particle size and coefficient of variation obtained according to the changes in operating conditions are as follows.

[0052] Table 1. Changes in principal grain size of crystals obtained with different seed amounts (45 μm)

[0053]

[0054] Table 2. Changes in principal grain size of crystals obtained with different relative supersaturation.

[0055]

[0056] Table 3. Changes in principal grain size of crystals obtained with different suspension densities.

[0057]

[0058] In a continuously operating crystallizer, each crystal product grows from a single crystal nucleus. Within a given crystal slurry volume, the greater the number of tiny crystal nuclei generated, the larger the product crystal will grow. During sodium nitrate crystallization, even after adding seed crystals, collisions between crystals or between crystals and the vessel wall will still generate a large number of fine crystals. A limited amount of solute in the solution will deposit on the surface of these excessively large crystal nuclei, inevitably resulting in a smaller product crystal size. To remove excessively large fine crystals from the crystal slurry and increase the average particle size of the product, the fine crystals can be removed by reheating and melting them before being recycled back into the crystallizer. The melted fine crystals then act as solute to increase the growth of the finished product, which is beneficial for the steady increase in particle size. The fine crystal removal recycling ratio R = 1 + QR / Q, where QR is the volumetric recycling rate for fine crystal removal, and Q is the product volumetric discharge rate. The crystallizer used in this invention is a DTB crystallizer, characterized by the presence of a clarification zone where fine crystals are suspended. The fine crystal removal recycling ratio is adjusted by controlling the fine crystal discharge flow rate in this clarification zone. In this invention, all three stages of flash evaporation cooling crystallization incorporate fine crystal elimination. Because the suspension density gradually increases during the stepwise crystallization of sodium nitrate, the collision frequency between crystals and between crystals and the vessel wall increases, resulting in a greater number of fine crystals. Therefore, the fine crystal elimination cycle ratio of the flash crystallizer should be increased stepwise. In this invention, the fine crystal elimination cycle ratio R = 2–3 for the first flash crystallizer, R = 3–3.5 for the second flash crystallizer, and R = 3.5–5 for the third flash crystallizer.

[0059] Table 4. Changes in principal grain size of crystals obtained with different grain refinement elimination cycles.

[0060]

[0061]

[0062] The fine-grain elimination ratio has a significant impact on the main particle size of sodium nitrate crystals. In the three-stage flash crystallization process, the main particle size of sodium nitrate crystals increases significantly after adding the fine-grain elimination operation. However, once the fine-grain elimination ratio R exceeds a certain value, the impact on the main particle size becomes minimal, and a large fine-grain circulation volume leads to additional heating costs. In this invention, the fine-grain elimination volume is determined based on economic trade-offs. The fine-grain elimination circulation ratio R in the first flash crystallizer is 2–3, in the second flash crystallizer it is 3–3.5, and in the third flash crystallizer it is 3.5–5. Following the fine-grain elimination ratio of this invention, large-particle sodium nitrate crystals with a D90 > 0.9 mm can be stably produced.

[0063] Although the embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the scope of the invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A production process for a multi-stage flash evaporation and cooling crystallization production system for large-particle sodium nitrate, characterized in that: Includes the following steps: Sodium nitrate solution heating and evaporation: The feed tank (1) is connected to the feed pump (2). The feed pump (2) transports the raw material liquid in the feed tank (1) into the circulation pipeline connected between the evaporation circulation pump (4) and the separator (5). The evaporation circulation pump (4) transports the mixed liquid into the evaporator (3) for heating. The heated sodium nitrate solution is then evaporated and concentrated to 62-65% in the separator (5). Adding seed crystals: The transfer pump (6) transports the concentrated sodium nitrate solution in the above-mentioned circulating pipeline to the seed crystal tank (7). The seed crystal tank (7) is equipped with a stirrer and continuously adds a certain amount and particle size of small seed crystals, wherein the seed crystal particle size is 20~50μm, the amount added is 0.2~0.4% of the mass of the above-mentioned concentrated sodium nitrate solution, and the residence time of the seed crystals in the seed crystal tank is 0.5~1h. First-stage flash crystallization: The suspension in the seed tank (7) is transported to the first flash crystallizer (8) by pressure difference. The evaporation temperature of the first flash crystallizer (8) is 60~70℃, the evaporation pressure is 20~32kPa, the suspension density is 120kg / m³, and the relative supersaturation of the sodium nitrate solution in the first flash crystallizer is 1~1.5%. Secondary flash crystallization: The crystal slurry in the first flash crystallizer (8) is fed into the second flash crystallizer (12) through the first transfer pump (11); the second flash crystallizer (12) has an evaporation temperature of 50~60℃, an evaporation pressure of 12~20kPa, a suspension density of 180kg / m³, and a relative supersaturation of sodium nitrate solution of 1.5~2.5% in the second flash crystallizer; Three-stage flash crystallization: The crystal slurry in the second flash crystallizer (12) is fed into the third flash crystallizer (16) through the second transfer pump (15). The evaporation temperature of the third flash crystallizer (16) is 40~50℃, the evaporation pressure is 7.5~12kPa, the suspension density is 300kg / m³, and the relative supersaturation of the sodium nitrate solution in the third flash crystallizer (16) is 2.5~4.0%. Thickening and separation: The crystal slurry in the third flash crystallizer (16) is transported to the thickener (24) by the third transfer pump (19). After thickening, the crystal slurry in the thickener (24) flows into the first centrifuge (25). The mother liquor from the thickener (24) and the first centrifuge (25) flows into the mother liquor tank (29). Centrifugal drying unit: After the crystal slurry is dehydrated by the first centrifuge (25), the moisture content of the wet salt is 3~4%. The wet salt is conveyed to the washing tank (26) by the screw feeder and washed with the raw material liquid for 0.25~0.5h. After washing, the crystal slurry flows into the second centrifuge (27) for dehydration. After dehydration, the wet salt is sent to the dryer (28) for drying. The particle size of the dried sodium nitrate crystals is 0.9~1.3mm, and the coefficient of variation CV≤25. Mother liquor reflux: The washing mother liquor flows into the feed tank (1) by gravity. The mother liquor stored in the mother liquor tank (29) is refluxed by the reflux pump (30) to a portion of the mother liquor and then to the second flash crystallizer (12) and the third flash crystallizer (16), respectively. The other portion of the mother liquor is refluxed back to the feed tank (1), and a mother liquor discharge pipeline is set up. The mother liquor reflux flow rate of the second flash crystallizer (12) is 20~25%, the mother liquor reflux flow rate of the third flash crystallizer (16) is 40~45%, the mother liquor reflux flow rate of the feed tank is 30~35%, and the mother liquor discharge rate is 0~5%. The large-particle-size sodium nitrate multi-stage flash cooling crystallization production system includes a feeding unit, an evaporation unit, a seed crystal unit, a flash crystallization unit, a condensation unit, a thickening unit, a centrifugal drying unit, and a mother liquor reflux unit. The feeding unit includes a feeding tank (1) and a feeding pump (2). The evaporation unit includes an evaporator (3) and an evaporation circulation pump (4). The feeding tank (1) is connected to the inlet of the evaporator (3) through the feeding pump (2) and the evaporation circulation pump (4). The outlet of the evaporator (3) is connected to the inlet of the separator (5). The seed crystal unit includes a transfer unit. Pump (6) and seed tank (7), the outlet of separator (5) is connected to seed tank (7) via transfer pump (6); the flash crystallization unit includes a first flash crystallizer (8), a first transfer pump (11), a second flash crystallizer (12), a second transfer pump (15), a third flash crystallizer (16) and a third transfer pump (19), the outlet of seed tank (7) is connected to the inlet of the first flash crystallizer (8), and the outlet of the first flash crystallizer (8) is connected to the second flash crystallizer (12) via the first transfer pump (11). The feed inlet of the first centrifuge (25) is connected to the feed outlet of the second flash crystallizer (12), and the discharge outlet of the second flash crystallizer (12) is connected to the feed inlet of the thickener (24) of the thickening unit via the second transfer pump (15). The discharge outlet of the third flash crystallizer (16) is connected to the feed inlet of the thickener (24) of the thickening unit via the third transfer pump (19). The discharge outlet of the thickener (24) is connected to the feed inlet of the first centrifuge (25). The centrifugal drying unit includes a washing tank, a second centrifuge (27), and a dryer (28). The discharge outlet of the first centrifuge (25) is connected to the washing tank, and the discharge outlet of the washing tank is connected to the feed inlet of the first centrifuge (28). The outlet of the second centrifuge (27) is connected to the dryer (28); the mother liquor reflux unit includes a mother liquor tank (29) and a reflux pump (30); the reflux ports of the first centrifuge (25) and the thickener (24) are connected to the mother liquor tank (29); the mother liquor tank (29) is connected to the second and third flash crystallizers of the flash crystallizer unit via the reflux pump (30); the gas outlets of the first flash crystallizer (8), the second flash crystallizer (12), and the third flash crystallizer (16) are connected to the condensation unit; The first flash crystallizer (8) has a clarification zone. The first fine crystal eliminator (9) is connected to the clarification zone of the first flash crystallizer (8). The suspension containing fine crystals in the clarification zone is heated and melted by the first fine crystal eliminator (9) and then returned to the first flash crystallizer (8) by the first fine crystal elimination pump (10). The particle size of the fine crystals entering the first fine crystal eliminator (9) from the clarification zone of the first flash crystallizer (8) is no greater than 30 μm, and the fine crystal elimination cycle ratio R = 2~3. The second flash crystallizer (12) has a clarification zone. The second fine crystal eliminator (13) is connected to the clarification zone of the second flash crystallizer (12). The suspension containing fine crystals in the clarification zone is heated and melted by the second fine crystal eliminator (13) and then returned to the second flash crystallizer (12) by the second fine crystal eliminator pump (14). The particle size of the fine crystals in the clarification zone of the second flash crystallizer (12) entering the second fine crystal eliminator (13) is no greater than 50 μm. The fine crystal elimination circulation ratio R = 3~3.

5. The third flash crystallizer (16) has a clarification zone. The third fine crystal eliminator (17) is connected to the clarification zone of the third flash crystallizer (16). The suspension containing fine crystals in the clarification zone is heated and melted by the third fine crystal eliminator (17) and then returned to the third flash crystallizer (16) by the third fine crystal eliminator pump (18). The particle size of the fine crystals entering the third fine crystal eliminator (17) from the clarification zone of the third flash crystallizer (16) is no greater than 80 μm. The fine crystal elimination cycle ratio R = 3.5~5.

2. The production process of the large-particle-size sodium nitrate multi-stage flash evaporation cooling crystallization production system according to claim 1, characterized in that: The condensation unit is a condenser (23). The gas outlets of the evaporation chambers of the first flash crystallizer (8), the second flash crystallizer (12), and the third flash crystallizer (16) are respectively equipped with a first pressure regulating valve (20), a second pressure regulating valve (21), and a third pressure regulating valve (22), which are connected to the condenser (23) via the first pressure regulating valve (20), the second pressure regulating valve (21), and the third pressure regulating valve (22).

3. The production process of the large-particle-size sodium nitrate multi-stage flash evaporation cooling crystallization production system according to claim 1, characterized in that: The first flash crystallizer (8), the second flash crystallizer (12), and the third flash crystallizer (16) are all DTB crystallizers.

Images